1. Field of the Invention
The invention relates to flame retardant, crosslinkable polyolefin-based compositions and articles produced therefrom. More specifically the compositions of the invention are comprised of an ethylene-silane polymer, an ethylene plastomer and a flame retardant filler. The compositions are used for the manufacture of films and sheets and are particularly useful for the production of roofing membranes.
2. Description of the Prior Art
Crosslinkable ethylene polymers having alkoxysilane functionality incorporated into the polymer either by grafting an unsaturated alkoxysilane onto an ethylene polymer backbone or directly copolymerizing ethylene with an unsaturated alkoxysilane are known. In the presence of moisture the alkoxysilane groups undergo hydrolysis and condensation reactions to form crosslinks. Silanol condensation catalysts are typically used to increase the rates of hydrolysis and condensation.
Crosslinkable ethylene polymers prepared by grafting unsaturated alkoxysilanes to ethylene polymers are disclosed in U.S. Pat. No. 3,646,155. Crosslinkable ethylene polymers prepared by copolymerizing ethylene with an unsaturated alkoxysilane are disclosed in U.S. Pat. Nos. 3,225,018 and 3,392,156.
Compositions of the above types are widely used for wire and cable insulation. For example, U.S. Pat. No. 6,197,864 discloses flame retardant insulation compositions comprised of 30 to 90 weight percent olefin-alkoxysilane copolymer derived from an α-olefin having from 2 to 8 carbon atoms and 0.25 to 20 percent by weight, based on the weight of the copolymer, of an unsaturated alkoxysilane of the formulaR*—Si(R**)a(Y)3-awherein R* is an ethylenically unsaturated hydrocarbon radical having from 2 to 6 carbon atoms, R** is a hydrocarbon radical having from 1 to 10 carbon atoms, Y is an alkoxy group having from 1 to 4 carbon atoms and a is an integer from 0 to 2; 5 to 40 weight percent halogenated organic compound; 1 to 20 weight percent antimony trioxide; 0.01 to 2.5 weight percent silanol condensation catalyst; and 1 to 40 weight percent magnesium hydroxide. Optional ingredients such as hindered phenol antioxidants and mineral fillers may also be included in the formulations.
Copending application Ser. No. 10/653,514, now U.S. Pat. No. 6,936,655, discloses wire and cable insulation compositions having improved abrasion resistance comprising 30 to 90 weight percent high density, bimodal silane-containing polyethylene base resin which can be a blend of a bimodal high density polyethylene resin having a density of 0.940 to 0.960 g/cm3, melt flow ratio from 50 to 300 and ratio of weight average molecular weight to number average molecular weight from 15 to 30 with an ethylene-silane copolymer or a bimodal high density polyethylene resin having a density of 0.940 to 0.960 g/cm3, melt flow ratio from 50 to 300 and ratio of weight average molecular weight to number average molecular weight from 15 to 30 grafted with a silane monomer; 5 to 70 weight percent flame retardant; and 0.01 to 1 weight percent silanol condensation catalyst.
Crosslinkable olefin-silane copolymers have also been utilized for roofing membrane materials. U.S. Pat. No. 4,722,961 discloses roofing membrane compositions comprising (a) a hydrolyzable polyolefin having a density less than or equal to 0.92 selected from the group consisting of (i) copolymers of ethylene and silane and (ii) silane modified polyethylenes wherein silane is present in an amount of about 0.5 to about 10 percent by weight based on the weight of the polyolefin, said polyolefin being present in the composition in an amount of about 20 to about 90 percent by weight based on the combined weight of components (a) and (b); (b) an ethylene-propylene-diene terpolymer rubber, said terpolymer rubber containing about 30 to about 85 percent by weight ethylene, about 15 to about 70 percent by weight propylene and about 1 to about 10 percent by weight diene, all percentages based on the weight of the terpolymer rubber, said terpolymer rubber being present in the composition in an amount of about 10 to about 80 percent by weight based on the combined weight of the components (a) and (b); (c) a particulate filler selected from the group consisting of carbon black and non-black reinforcing fillers, and mixtures thereof, said filler being present in the composition in an amount of about 10 to about 200 parts by weight per hundred parts by weight of components (a) and (b) combined; and (d) a compound selected from the group consisting of paraffin oils, naphthenic oils, and liquid polybutene, said compound being present in the composition in an amount of about 10 to about 100 parts by weight per hundred parts by weight of components (a) and (b).
Roofing membrane compositions derived from propylene polymers are disclosed by N. Dharmarajan, et al., in an article entitled “Metallocene Plastomer Based Thermoplastic Olefin Compounds Designed for Roof Membrane Applications,” ANTEC 2001, pages 1694-1698 and in an article by T. A. Glogovsky, et al., entitled “Development and Characterization of a New Plastomer Designed for Single-Ply Roofing Membranes,” ANTEC 2002 Plastics: Annual Technical Conference, Vol. 3: pg. 189. The compositions of the latter reference utilize ethylene-octene-1 plastomers produced using metallocene catalysts as one of the components with the propylene polymer.
Membranes produced in accordance with the above-referenced disclosures are deficient in flame retardance. Furthermore, the propylene polymer based compositions are not crosslinked which limits their utility to lower temperature membrane applications.
There is a continuing need for compositions which can be used for single-ply roofing membranes or multi-layer membrane constructions and particularly for crosslinkable, flame retardant, tear and puncture resistant compositions capable of withstanding prolonged exposure to the elements, i.e., wind, rain, sun, high and low temperatures, etc. Additionally, it would be even more advantageous if membranes formed from such compositions had good flexibility and heat sealability for ease of application. Heat sealability, i.e., the ability to seal overlapping material without the use of adhesives, is highly desirable since roofing membranes are typically installed as wide (6-12 foot) sheets which are overlapped and preferably heat welded to provide strong, watertight bonds between adjoining sheets. These and other advantages are achieved with the compositions of the invention.